Multiplying matrices with different parents?

I want to conjugate a symbolic matrix, Sigma, by a matrix, garbage, over Z/9Z.
If I define both matrices as symbolic matrices, I get the right answer. If I define garbage over Z/9Z, I get confusing answers. Can anyone explain my results?

Comments

1

Thank you for looking into this for me! As a new user, I'm glad to know it wasn't something ridiculous on my part.
Also, thanks for the suggestion to use the polynomial ring. That will get me through what I need to do.

Thierry, if you can get any smaller examples of this please do open a ticket. But I agree that in general SR is ill-equipped to handle modular stuff, because SR sort of implicitly assumes characteristic zero I think.